In order to avoid the processing problems and to meet product quality specifications as mentioned above, it is common practice to hydrotreat various stocks for removal of sulfur, nitrogen and metals. For example, feed for hydrocracking may be first contacted with a hydrotreating catalyst in the presence of hydrogen. The hydrotreater effluent is condensed and separated from unused hydrogen, ammonia, hydrogen sulfide and gaseous hydrocarbons such as methane for recycle to the reactor after scrubbing to remove hydrogen sulfide and ammonia. The condensate is then mixed with a further supply of hydrogen and passed through one or more beds of hydrocracking catalyst to produce products of lower boiling range than the feed. Typically, the hydrocracker is a series of beds in a vertical reactor and the charge is passed downward in concurrent flow with hydrogen. The reactions taking place are exothermic, resulting in a temperature rise in each bed. Temperature is controlled by addition of cold hydrogen between the beds.
It will be seen that the conventional hydrocracker is a multi-stage operation of first stage pretreater and second stage hydrocracker with similar reactions taking place in both stages, but to different relative degrees. In the first stage, the predominant reactions are desulfurization, denitrogenation and demetallation with a lesser degree of cracking. The high pressure separator to provide a recycle hydrogen stream will remove methane but, for the most part, other cracked products will be retained in the feed to the hydrocracker. In the latter stage, the predominant reaction is cracking, applied alike to heavy components and to potential gasoline components derived from cracking in the first stage. Such nitrogen and sulfur compounds as remain after the first stage will be subjected to conversion reactions for removal of these contaminants.
In an effort to reduce the capital cost and operating expense of multi-stage operation, it has been proposed that the first stage (pretreater) effluent be cascaded to the second stage. This results in supply to the hydrocracker of all components of the first stage effluent.
An interesting variant on hydrotreating residual stocks is described in Franz et al. U.S. Pat. No. 3,897,329. The feed is introduced to a region intermediate two beds of cobalt-moly on alumina catalyst. Hydrogen is added with the feed. In addition, hydrogen is supplied to the bottom of the lower bed to pass countercurrent to liquid hydrocarbons flowing down through the bed. In that lower bed, desulfurization takes place at about 850.degree. F. Vaporous products from reaction in the lower bed and those present in the charge pass up into the upper bed together with hydrogen and are there further reacted at the higher temperature of 875.degree. F. The higher temperature in the upper bed will be effective to avoid condensation in that upper bed and return of reflux to the lower bed. As pointed out in the patent, it is not necessary that the two beds be in the same chamber, only that there be conduit means for conveying vapor and hydrogen from the lower bed to the upper bed. In effect, these patentees are providing for more severe reaction conditions (higher temperature and greater hydrogen concentration) applied to the vapor phase charge in the upper bed.
In essence, the Franz et al. patent describes a method of connecting a concurrent vapor phase reactor with a countercurrent trickle bed or mixed phase reactor without interstage separation.
Another form of multiple bed hydrotreating with intermediate supply of charge is found in Pappas et al. U.S. Pat. No. 3,091,586. Contrary to the effects of Franz et al., the Pappas et al. system provides for greatest severity of treatment for the liquid fraction which passes downwardly through three successive catalyst beds. Overcracking of vaporous products formed in any of these beds is avoided by withdrawal of the gas phase from a space above each bed. As a consequence, a fresh supply of hydrogen must be introduced to the bottom of each such bed. A somewhat similar effect is obtained in Scott, Jr. U.S. Pat. No. 3,425,810 by multiple feed and withdrawal conduits in a multi-bed hydrotreater. See also Halik et al. U.S. Pat. No. 3,211,641.
The Pappas et al. patent describes a method of connecting several countercurrent trickle bed reactors with a concurrent vapor phase reactor in the hydrofining of shale oil. Means are provided between reactors to allow the addition of hydrogen and the withdrawal of vaporous products and to conduct liquid flowing from one reactor to the reactor below. It is noted that the vaporous product from one reactor is not fed to the next reactor. Other than the specific mechanical connections, the system is not different from an installation of multistage reactors all placed on the ground level.
Scott's hydrotreating apparatus is basically a series of countercurrent flow reactors stacked vertically, with provisions for adding and withdrawing vapor and liquid streams from each reactor.
The fact that the reactors are stacked vertically does not differ in principle from multistage reactors on the ground level except that interstage circulation pumps must be provided for the ground level facility.
The Halik et al. patent describes the addition to a multistage-reactor system, of a confined saturation zone to dissolve hydrogen in the liquid feedstock and a lift tube which allows the liquid reactant to be recycled from the bottom of the reactor to the top of the reactor with a gaseous reactant stream (hydrogen).
Greater severity of treatment for the liquid portion of a hydrofining charge is provided in Wilson et al. U.S. Pat. No. 3,658,681. That system puts the charge through vacuum distillation to yield a vacuum overhead fraction passed downwardly through a top bed of a reactor to mix with the product of passing the vacuum bottoms upwardly through three beds of catalyst. The combined effluent is withdrawn as a single product stream. Although the Wilson et al. arrangement does afford the advantage of greater severity of treatment for the fraction needing the greater severity, it achieves that result in a rather cumbersome style, involving a pretreatment vacuum distillation and a post-treatment distillation to separate distillate and residual fuels.